Building officers inspected the tenement in To Kwa Wan twice in November and December before it collapsed but identified no structural risk, the Buildings Department said.

The department's revelation came as a woman, purported to be the owner of the collapsed block at 45J Ma Tau Wai Road, said she had invited officers to check the block before removing illegal structures last month.

A woman, identifying herself as Chak Oi-luen, who, according to the Companies Registry, owns block J through two companies, told Next Magazine that she wanted to apologise to families of those killed in the collapse of the 55-year-old, five-storey building.

Block J became rubble in less than 20 seconds last Friday, killing four people and leaving dozens homeless.

Chak said she had relied on an accountancy firm and two estate agencies to take care of units on the top four floors.

She said she was ignorant of how the units were managed, including how they were subdivided.

The ground-floor shop was under her charge. After building officers examined her block, she hired a contractor named Chu to remove illegal structures and renovate the ground-floor shop unit last month. The building collapsed during the fourth day of Chu's work.

But Chu had said he had not removed any load-bearing columns, the report said. Four load-bearing columns in the shop were 'broken' before renovation started, he said, adding that he removed only an air-conditioning vent and boards.

The South China Morning Post yesterday went to the Ho Man Tin home address of Chak Oi-luen, as stated in the Companies Registry. A young woman answered the door but denied Chak lived there and said no one there was in charge of the block that had collapsed.

Wai Wing Construction Decoration, which Chak said was Chu's company, could not be located. The firm is not in the Companies Registry, nor is it on the list of the Buildings Department's 'authorised persons' who can carry out building works.

The department confirmed that officers had been sent to inspect 45J Ma Tau Wai Road in November and December.

'According to our inspection, the building concerned was structurally safe,' a spokeswoman said.

It could not confirm whether officers had entered block J units to check if partitioning had weakened the structure, she said.

The spokeswoman said the department had never received an application from the building's owner to subdivide the flats into small suites - which tenement owners commonly do to earn higher rental income - although such applications are required.

Asked if the department had underestimated risks to the building, the department said it always attached great importance to building safety. 'The department's building surveyors and structural engineers are competent professionals, having both the professional qualifications and ample experience in building inspection to determine whether the structure is safe,' she said.

Police and buildings officials continued their investigation of the collapse yesterday.

A government spokesman said last night that white asbestos had been found in three of seven tile samples taken from the remains of block J. The amount of asbestos - a construction material now banned because some types of asbestos can cause lung cancer - was estimated to be very small, the spokesman said. White asbestos was low risk and the tiles would not emit dust harmful to health, he said.

Residents in the rest of the cluster at 45 Ma Tau Wai Road said cracks had started to develop in beams inside their units.

Pou Yiu-keung, who lives in a partitioned room on an enclosed balcony in block D, said he was sceptical about buildings officials' assurance that his block was safe.

'Since the accident, officers have only come to check the conditions in common areas like the staircase,' Pou said. 'They never came in to see the many new cracks on the beams.'

A man named Mak living in the same block also said a 1cm crack had become apparent on a beam after the accident. He said he would not pay his rent this month because he was afraid he would soon be evicted.